Why can I download Kindle books from amazon.com but not from amazon.co.uk?

I installed the **Android Kindle** application and purchased my first Kindle e-book yesterday. Although I have an account on **amazon.co.uk** and have used this account for multiple purchases of dvd's and regular books, I **cannot purchase Kindle e-books** from this site. **I can, however, purchase Kindle e-books from amazon.com** (with the same login). Does anyone know why this is ?